The "No one ages" thing is for Disney policies regarding higher ticketing, room occupancy, and dining (plan?) charges (unsure if a 2 year old will be free at CM if they turn 3 before the visit during that trip and paying OOP).
For example, if you check in with a 2 year old, and their birthday is on day 3 of your trip, they are 2 with regards for those 3 points above for the entire trip. Same deal with 9 year olds, and (possibly?) same deal with 17 year old with regards to extra adult charges at the resorts.
Since the drinking age doesn't fall under these policies, you can get a drink just fine if you turn 21 during a trip.